The present invention relates to the transmission of data and, in particular, to the transmission of data over voiceband telephone channels using modems.
The telephone network of today is evolving toward the almost exclusive use of digital carrier systems. At the heart of such systems are non-linear analog-to-digital (A/D) converters which encode analog voice and modem signals for digital transmission. For example, the common D4 channel bank uses 64 kilobit per second (KBPS) pulse code modulation (PCM). In addition, many systems specifically engineered for voice transmission use 32 KBPS adaptive differential PCM, or ADPCM. The channel capacity of these two systems is--almost by definition--64 and 32 KBPS, respectively.
It is desirable to be able to use that channel capacity to the maximum possible extent in transmitting data over the voiceband telephone network using analog modems. It has been found, however, that the non-linearities in such systems--particularly the non-linearity deliberately engineered into the A/D converters--serves as a limit on the maximum bit rate that analog modems can obtain while still maintaining a commercially acceptable error rate. This limitation is principally due to the multiplicative effect that the non-linearities have on the noise in the channel. For PCM systems, for example, the presently achievable maximum rate using standard, trellis coding technology appears to be no more than about 28.8 KBPS, notwithstanding the theroretical capability of 64 KBPS.
The prior art has recognized this phenomenon. See, for example, K. Pahlavan et al, "A Method to Counteract the Effects of PCM Systems on the Performance of Ultra High Speed Voice-Band Modems," IEEE International Conference on Communications, 1986, Vol. 3, pp. 1592-6, and U.S. Pat. No. 4,660,214 issued Apr. 21, 1987, both of which are hereby incorporated by reference. The Pahlavan et al paper and patent also propose a technique for dealing with the problem. At the heart of that technique is the use of a signal constellation (or "signal structure") whose signal points are non-uniformly spaced in such a way as to compensate for increasing levels of noise associated with signal points that are increasingly distant from the origin. In a specific embodiment of a trellis-coded system which implements their technique, Pahlavan et al use a) a pair of equations derived from a mathematical model of the multiplicative noise phenomenon to position each point in the constellation in conjunction with b) a modification to the distance measurement relation used in the Viterbi decoder. See, in particular, column 8, line 35 to column 9, line 63 of the Pahlavan et al patent.